Photovoltaic systems are increasingly used to supply electric power. For example, many buildings include rooftop photovoltaic systems for supplying some or all of the building's electric power. As another example, electric utilities have built large photovoltaic systems, sometimes referred to as solar “farms,” for supplying electric power to large numbers of customers.
A single photovoltaic cell typically generates electric power at less than one volt. Many electric power applications, however, require voltages that are much higher than one volt. For example, inverters powered by photovoltaic systems often require input voltages of several hundred volts. Therefore, many photovoltaic systems include a large number photovoltaic cells electrically coupled in series to obtain a sufficiently high voltage for their application. Additionally, many photovoltaic systems include two or more strings of photovoltaic devices electrically coupled in parallel to achieve a desired system power generation capacity.
FIG. 1 illustrates a prior art photovoltaic system 100 including a first string 102 electrically coupled in parallel with a second string 104. String 102 includes M photovoltaic devices 106 electrically coupled in series, and string 104 includes N photovoltaic devices 108 electrically coupled in series, where M and N are each positive integers greater than one. In this document, specific instances of an item may be referred to by use of a numeral in parentheses (e.g., photovoltaic device 106(1)) while numerals without parentheses refer to any such item (e.g., photovoltaic devices 106). Photovoltaic devices 106, 108 are either individual photovoltaic cells or groups of electrically coupled photovoltaic cells. First and second strings 102, 104 are electrically coupled in parallel with a load 110.
High voltages may exist in many photovoltaic systems. For example, each string 102, 104 of photovoltaic system 100 will often include many series-coupled photovoltaic cells, such that voltage across power rails 112, 114 will often exceed one hundred volts, especially in systems coupled through inverters to alternating current (AC) power grids. Indeed, photovoltaic systems are often rated at 600 volts or 1,000 volts. Additionally, many photovoltaic systems are capable of supplying significant current. Accordingly, photovoltaic systems may experience an electrical are, where gas (typically air) between two nearby nodes ionizes due to a large voltage between the nodes, resulting in current flow between the nodes. Such potential for an electrical arc is compounded by the fact that typical photovoltaic systems include many electrical connectors and long electrical cables, thereby presenting many possible points of failure. Additionally, photovoltaic systems are often subjected to hostile environmental conditions, such as extreme temperatures and intense ultraviolet radiation, which may cause connector or insulation failure, particularly over the long lifetimes expected of typical photovoltaic systems. Furthermore, some photovoltaic systems are vulnerable to physical damage, such as from maintenance personnel working in the system's vicinity, or from an animal chewing on the system's components.
A photovoltaic system electrical arc can be classified as either a series electrical arc or a parallel electrical arc. A series electrical arc occurs across an opening in a series electrical circuit, such as across an opening caused by a connector failure. For example, FIG. 2 illustrates a series electrical arc 202 across an opening 204 in first string 102 of photovoltaic system 100. A parallel electrical arc occurs between two nodes of a photovoltaic system, or between a node and ground, such as due to an insulation failure. FIG. 3 illustrates a parallel electrical arc 302 between a node 116 of second string 104 and negative power rail 114 of photovoltaic system 100.
Photovoltaic system electrical arcs are usually highly undesirable because their heat can injure a person or animal in the system's vicinity, start a fire, damage the photovoltaic system, and/or generate electrical noise which can disrupt proper operation of nearby electrical circuitry. Additionally, an energized photovoltaic system may present an electrical shock hazard to firefighters attending to an arc-induced fire. Consequently, arc fault detectors and arc fault circuit interrupters have been developed for photovoltaic systems. An arc fault detector (AFD) detects occurrence of an electrical arc in a photovoltaic system, and an arc fault circuit interrupter (AFCI) opens an electrical circuit of the photovoltaic system in response to the detected electrical arc, to help de-energize the photovoltaic system. In some cases, an AFD and an AFCI are combined into a single device which is capable of detecting occurrence of an electrical arc and opening an electrical circuit in response thereto.
AFDs typically detect occurrence of an electrical arc by identifying high frequency components, or “noise,” of photovoltaic system current that is generated by the electrical arc. The noise's amplitude is very small and must be increased by amplification, or by use of a current transformer, for detection. Additionally, the noise must be distinguished from other high frequency components commonly present in photovoltaic system current, such as switching power converter ripple current and harmonics thereof. Thus, AFDs typically decompose photovoltaic system current into its constituent AC components using Fast Fourier Transform (FFT) techniques, or similar frequency detection techniques, to distinguish electrical arc noise from other system noise.
It is sometimes necessary to completely de-energize a photovoltaic system to extinguish an electrical arc. Additionally, it is generally desirable to completely shut down a photovoltaic system upon an occurrence of an electrical arc to promote safety and to minimize equipment damage risk. Furthermore, a given AFD is often only able to detect electrical arcs occurring near the AFD, because electrical arc noise is significantly attenuated as it travels through the photovoltaic system. Accordingly, some photovoltaic systems include distributed AFDs and AFCIs, such as within each photovoltaic module, to maximize likelihood that an electrical arc occurrence will be detected, and to enable the photovoltaic system to be completely shut down upon detection of the electrical arc occurrence.
Conventional photovoltaic systems including an AFD and AFCI with each photovoltaic assembly require communication among photovoltaic assemblies and/or communication with a central management device to ensure complete photovoltaic system shutdown in response to detection of an electrical arc occurrence, and/or to ensure complete photovoltaic system shutdown for maintenance or other periods of non-use. For example, consider a scenario where an electrical arc occurs near an edge of a photovoltaic system. An AFD of a photovoltaic assembly distant from the system edge may not detect the electrical arc due to attenuation of its associated electrical noise. Consequentially, an AFD near the system edge must communicate detection of the electrical arc occurrence to the distant photovoltaic assembly, or to a central management device capable of shutting down the entire photovoltaic system, to ensure that the distant photovoltaic assembly shuts down in response to detection of the electrical arc occurrence. Such communication among photovoltaic assemblies is conventionally achieved, for example, by a wired communication network, a wireless communication network, an optical communication network, or a power line communication network.